It is known in the art that a cannula for use, particularly a single use, of transferring fluid into or from objects, in particular living beings, can be moulded from polymeric materials. Stevens, Smith and Bartlett described a method for manufacturing hypodermic needles in U.S. Pat. No. 5,620,639, hereinafter referred to as the '639 patent, which is incorporated herein by reference. The method disclosed in the '639 patent advantageously uses the properties of a liquid polymer and a working fluid for displacing liquid polymer from the interior of channel in a mould containing liquid polymer. The working fluid of the '639 patent is advantageously a gas that reliably creates an integrated “needle” comprising a cannula portion and a mounting hub portion for connection to a syringe, the cannula defining a conduit for passage of fluids between a reservoir and a subject. The mounting hub portion of a needle can take many forms, including being an extension of the needle portion. The cannula comprises of a conduit portion, a holding portion and a distal portion or entry portion, the cannula incorporating apertures known as ports for delivery of fluids. The holding portion may have the function of providing a means for holding the cannula as well as other functions such as, but not limited to, providing a reservoir for the fluid for delivery. While needles are cannulae that are characteristically used for transferring fluids in medical applications, there are many different types of articles that function as cannulae in many applications.
Methods for using fluid-assisted injection moulding of such small articles as cannulae rely on very precise metering of the volume of liquid polymer injected into the small constrained cavities required in the closed dies or moulds used for forming the articles. On the one hand, injecting too little volume of liquid polymer, for example, can result in an incomplete article. On the other hand, injecting too much volume of liquid polymer may lead to the working fluid not displacing enough of the liquid interior portion of the polymer to form a continuous conduit in the cannula. In either example, the formed article may be unsuitable for its intended use and must be discarded because it does not meet performance specifications, particularly in medical applications.
The method of the '639 patent has been shown to be effective using a single shot of liquid polymer in a mould containing a small number of cavities, say, 4 to 8, to each cavity producing a cannula in each cavity. However, large scale production of cannulae is necessary to be cost-effective and cheap relative to other methods of producing cannulae known in the art. Ideally, a mould for injection moulding of cannulae includes many cavities, each formed in the desired shape of a cannula so that a single cycle of injection (a single shot) of liquid polymer into the mould will produce many correctly formed cannulae, each meeting performance specifications. Other methods of injection moulding of cannulae are known, such as that taught in U.S. Pat. No. 6,767,496. All methods have shortcomings in controlling the flow of polymer.
A convenient way to produce many cannulae from a single shot is to inject the liquid polymer at a central feed point of a mould, which is in fluid communication with multiple runner feeds, each runner feeding to one of a series of cavities, each cavity having the shape of a cannula. It is known in the art that a slight excess of liquid polymer can be injected to a cavity, the excess material being allowed to exit the cavity at an overflow, the overflow sometimes called a “spillover” that is open to the atmosphere or presents very little resistance to flow.
Liquid polymers have very high viscosities. Very high pressures are required for injection moulding and, in fluid-assisted injection moulding of polymers, the working fluid must also be forced under high pressure into the mould for it to achieve the desired result of pushing more liquid polymer through the solidified polymer layer and into the overflow until the pressurised working fluid itself enters the overflow to create a through-conduit. Characteristically, the highly pressurised working fluid on reaching such an overflow undergoes a rush in velocity as it passes out of the liquid polymer. This method is not suitable for multiple channels in a mould. If there are many cavities in a mould the sudden rush of pressurised working fluid out of the polymer can destroy the cannulae at the worst, or result in a large proportion of defective cannulae. This method could result in mechanical damage to surfaces of the cannulae, the damage being observable under microscopic observation. Such damage could cause contamination if loose microscopic material was accidentally delivered to a subject or collected from a fluid source.
To achieve the required specifications of many cannulae produced in a single moulding cycle using the methods of the prior art for fluid-assisted injection moulding, the shot size (the total amount of liquid polymer injected) must be matched with the total volume of all the cavities in the mould. For example, a general purpose hypodermic needle will require about 100 mg per cannula with a mounting hub. Therefore a mould with 32 cavities, would require a shot of about 3 g of liquid polymer plus the feed system that traps some material. Reliability of the process requires that the amount of liquid polymer injected into each cavity must be controlled to a few milligrams, and the variability in total volume of liquid polymer from one shot to the next should be less than 1% to ensure each cavity always receives a suitable amount of liquid polymer to ensure the best product quality. Such tight tolerances in the amount of liquid polymer delivered using the current methods means that it is difficult to achieve the desired product consistency.
What is needed is a method and apparatus for moulding cannulae, the method and apparatus having improved reliability in reproducing the fine details in the cavities in a mould, particularly near the distal portion of the cannula.